The instant invention pertains to semiconductor device fabrication and processing and more specifically to a method of stabilizing peroxygen-containing slurries in chemical mechanical planarization.
Two trends present in the semiconductor device manufacturing arena involve the desire to have planar structures and reducing contamination of structures overlying the semiconductor substrate. Chemical-mechanical polishing (CMP) is being utilized with greater and greater frequency to achieve both of these trends. Most semiconductor manufacturers use CMP to planarize dielectric layers and to planarize some metal structures. The most common approach used in the CMP process is to attach a semiconductor wafer to a carrier (which may or may not rotate) via a mounting pad and polishing the exposed surface of the wafer by bringing it into contact with a polishing pad (which is mounted on a rotating or non-rotating platen). The mechanical abrasion between the wafer surface and the polishing pad results in the polishing of the wafer surface. To aid in the polishing and the removal of any particles liberated in this process, a slurry is usually introduced between the wafer surface and the polishing pad. The chemical components of the slurry will react with the wafer surface, thereby making the wafer more easily polishable, and the excess slurry will carry away the materials liberated from the wafer during this polishing step.
Most processes used in the semiconductor industry, today, involve the formation of a layer of either conductive or insulating material by the planarization (typically CMP) of the material. For example, a dielectric layer, which is quite thick in comparison to other structures, is typically deposited over the entire wafer. Due to the underlying topology, the upper surface of this layer varies in height by a fairly appreciable amount. Using present lithography methods and equipment, the via holes/trenches that are to be patterned in this layer to form the conductive interconnects, are difficult to fabricate because of the variance in the height of this upper surface. Hence, a planarization step is required prior to the patterning of these holes. This step is, typically, accomplished by CMP, which results in a relatively planar upper surface of the dielectric layer.
In addition, CMP can be utilized after the blanket formation of a conductive material (such as tungsten, copper, aluminum, or other metal/combination of metals) so as to form inlaid conductive structures such as vias and/or interconnects. The use of CMP, as opposed to, for example, reactive ion etch (xe2x80x9cRIExe2x80x9d), is both cleaner and results in a more planar structure. Typically, metallization schemes are formed using CMP by first forming the openings in a dielectric layer for the vias and/or interconnects and then blanketly (or selectively) forming the conductive material so as to fill up the vias and/or interconnect openings. In order to properly fill these openings, an excess amount of the conductor is preferably deposited over the wafer. This results in the formation of a layer of conductive material over the dielectric layer in regions other than the openings. In order to remove this excess material, a CMP process is performed.
In some metallization schemes, the efficiency of the removal of the excess conductive material (or dielectric material) may be enhanced by introducing an oxidant to the CMP process so that a portion of the material to be removed becomes at least partially oxidized. The reason for this is that some oxidized materials are softer, and therefore more easily removed than the material itself. Furthermore, the oxidation of the top layer prevents the bulk of the material from getting oxidized and therefore control over planarization is made possible. For example, tungsten oxide is softer and more easily removed than tungsten. Hence, some semiconductor device manufacturers add hydrogen peroxide or other oxidizing agents to the slurry when polishing tungsten so as to cause the oxidation of portions (usually the peaks of the tungsten, structure) of the tungsten thereby making these portions easier to remove. The peroxide may be added to the slurry prior to its use to polish the layer of material to be removed or it may be added separately onto the pad. However it is preferable to mix the hydrogen peroxide into the slurry, in a ratio of around 1 to 10 percent by weight, prior to the delivery of the slurry to the CMP tool because of increased control and operational simplicity. See John P. Bare and Budge Johl, Evaluation of Manufacturing Handling Characteristics of Hydrogen Peroxide-Based Tungsten CMP Slurry, IEEE/CPMT INTERNATIONAL ELECTRONICS MANUFACTURING TECHNOLOGY SYMPOSIUM 164-171 (1998) However, there is problem with this method. Peroxygen-containing slurries have a short pot life. A common observation in the industry is that hydrogen peroxide-containing slurries decompose at a rate of around 0.1 wt % per day in a static bath which means that the slurry becomes less effective each day. Curve 102 of FIG. 1 illustrates the decomposition of hydrogen peroxide in a typical commercially available tungsten polish slurry. The result of this decomposition of the hydrogen peroxide is that the removal rate of the CMP process which uses this slurry diminishes at a rate which is not completely predictable. Ultimately, the removal rate will become so low that the slurry will no longer be usable and should be discarded.
This decomposition phenomena may be attributed to several different reasons. UV radiation impinging upon the slurry, elevated temperatures, and the high-pH of some slurries have been recognized by many in the industry as the major factors controlling the decomposition of hydrogen peroxide mixed into the slurry. However, the inventors of the instant application discovered that the presence of transition metals in the slurry can be the controlling cause of the decomposition of the peroxide in the slurry. The transition metals in the slurry will rapidly catalyze the decomposition of hydrogen peroxide, thereby forming water and oxygen as the by products.
An embodiment of the instant invention is a method of fabricating an electrical device having a structure overlying a semiconductor substrate which is planarized using chemical mechanical planarization, the method comprising the steps of: forming a layer of material over the semiconductor wafer; polishing the layer of material by subjecting it to a polishing pad and a slurry which includes peroxygen; and wherein the slurry additionally includes a stabilizing agent which retards the decomposition of the peroxygen in the slurry. Preferably, the stabilizing agent is comprised of: pyrophosphoric acids, polyphosphonic acids, polyphosphoric acids, Ethylenediamine Tetraacetic acid, a salt of the pyrophosphoric acids, a salt of the polyphosphonic acids, a salt of the polyphosphoric acids, a salt of the Ethylenediamine Tetraacetic acid and any combination thereof. In addition, the stabilizing agent may be comprised of: sodium pyrophosphate decahydrate, sodium pyrophosphate decahydrate, and/or 8-hydroxyquinoline. The decomposition of the peroxygen in the slurry is catalyzed by transition metals included in the slurry, and may be caused by the pH of the slurry. The layer of material is, preferably, comprised of: tungsten, copper, aluminum, a dielectric material, and any combination thereof.
Another embodiment of the instant invention is a method of chemical-mechanical polishing a layer situated over a semiconductor substrate, the method comprising the steps of: mixing a peroxygen-containing substance and a stabilizing agent with a slurry to create a slurry mixture; storing the slurry mixture for a period of time; using the slurry mixture with a chemical-mechanical polishing apparatus to planarize the layer situated over the semiconductor substrate; and wherein the stabilizing agent is comprised of a chemical selected from the group consisting of: sodium pyrophosphate decahydrate; pyrophosphoric acids, polyphosphonic acids, polyphosphoric acids, Ethylenediamine Tetraacetic acid, a salt of the pyrophosphoric acids, a salt of the polyphosphonic acids, a salt of the polyphosphoric acids, a salt of the Ethylenediamine Tetraacetic acid; 8-hydroxyquinoline and any combination thereof. Preferably, the stabilizing agent further comprises sodium pyrophosphate decahydrate.